Investigating adsorption and removal of divalent Ca2+ and Pb2+ ions from aqueous solutions by gamma-irradiation using quartz tuning fork (QTF) sensor technique

In this study, gold-coated quartz tuning forks (QTFs) sensing devices functionalized with self-assembled monolayers (SAM) of a lower-rim functionalized calix[4]arene methoxy ester were used for the detection of divalent Ca²⁺ and Pb²⁺ ions in aqueous solutions by utilizing adsorption behavior and the radiative effect. The gold-coated QTF functionalized calix[4]arene methoxy ester sensing device was tested by measuring the respective frequency shifts obtained using small (60 µL) samples of aqueous PbCl₂ at two different concentrations (10⁻⁶ and 10⁻⁴ M). For 10⁻⁴ M solutions of PbCl₂, results showed that the resonance frequency shift Δf = 317 Hz, from 32,867 Hz (f_Calix) to 32,550 Hz (f_{Calix⊃Pb²⁺}) due to the absorption of lead (Pb²⁺) ions (10^-4 M) by calixl[4]arene methoxy ester receptor molecules on the QTF sensing layer from the aqueous solution to forming the ([Calix ⊃ Pb²⁺]) complex. The most significant frequency changes were observed at a concentration of 10⁻⁶ M CaCl₂, where CaCl₂ exhibited the biggest change of 356 Hz, from 32,893 Hz (f_Calix) to 32,537 Hz (f_{Calix⊃Ca²⁺}), compared to 317 Hz for PbCl₂ (10⁻⁴ M). The limit of detection was 100 femtomolar (fM) for CaCl₂ and 245 fM for PbCl₂. After that, we irradiated the receptor molecules which was holding Pb²⁺ ions in the complex ([calix ⊃ Pb²⁺]) on the QTF sensing layer with a radiation dose ranging from 7.5 to 50 µGy of gamma rays from the Cesium-137 source for 30 min. Interestingly, it was observed that the resonance frequency shift (Δf = 54 Hz) back to 32,604 Hz from 32,550 Hz (f_{(Calix⊃Pb²⁺)}), which strongly suggests that the Pb²⁺ ion removed from ([calix ⊃ Pb²⁺]) complex on the QTF sensing layer due to gamma radiation dose. To follow up on the radiation effect of the ([calix ⊃ Pb²⁺]) on the QTF sensing layer, we stopped the gamma radiation source and kept it for an additional 10 min to see if there was any resonance frequency. It was noticed that an additional resonance frequency shifted (Δf = 33 Hz) back to 32,637 Hz from 32,604 Hz after stopping the gamma radiation source for 10 min. We assume that the complex ([calix ⊃ Pb²⁺]) absorbs the gamma radiation and continues the removal of Pb²⁺ ions from the complex on the sensing layer. A similar phenomenon was also observed for the absorption of lead (Pb²⁺) ions (10^-6 M) by calix[4]arene methoxy ester receptor molecules on the QTF sensing layer from the aqueous solution to forming the ([Calix ⊃ Pb²⁺]) complex. The resonance frequency shift Δf = 142 Hz, from 32,706 Hz (f_Calix) to 32,564 Hz (f_{Calix⊃Pb²⁺}) due to the absorption of lead (Pb²⁺) ions (10^-6 M) by calix[4]arene methoxy ester receptor molecules on the QTF sensing layer from the aqueous solution to forming the ([Calix ⊃ Pb²⁺]) complex. Subsequent exposure to gamma radiation, with doses ranging from 7.5 to 50 µGy from a Cesium-137 source over 30 min, prompted a resonance frequency shift (Δf = 37 Hz) back to 32,606 Hz from 32,564 Hz (f_{(Calix⊃Pb²⁺)}). This shift strongly suggests the removal of Pb²⁺ ions from the ([calix ⊃ Pb²⁺]) complex on the QTF sensing layer due to gamma radiation dose. X-ray photoelectron spectroscopy (XPS) analysis confirmed the chemical adsorption of Pb²⁺ ions onto the gold-coated QTFs functionalized calix[4]arene adsorbent sensing layer surface. Therefore, the technology underlying the calix[4]arene-functionalized sensing device holds promise for diverse industrial applications, supporting potential advancements in water pollution mitigation through the proposed adsorption and irradiation mechanism.